INTEGRATION OF SEISMIC PROFILES WITH OBSERVATIONS FROM THE SITE

Three multichannel seismic reflection profiles were obtained across the site before Leg 149 (Fig. 3, Fig. 5, and Fig. 6). These profiles show a number of reflectors that have been recognized on a regional scale in the vicinity of the Iberia Abyssal Plain and have been dated by tracing them back to Leg 103 sites west of Galicia Bank (Mauffret and Montadert, 1988) and to Site 398 near Vigo Seamount (Groupe Galice, 1979). Sonne Line 75-16 crosses the site normal to the basement ridge and best shows the characteristics of these reflectors and the intervening acoustic formations 1A, 1B, 2, and 3 that were defined by Groupe Galice (1979) (Fig. 3). The upper reflector corresponds to the 1A/1B boundary of Groupe Galice (1979) and marks a regional unconformity produced by the folding that accompanied the Rif-Betic compressional episode to the south. The middle reflector corresponds to the formation 1B/2 boundary (approximately Eocene/Oligocene) and the lowest reflector to the formation 2/3 boundary (approximately Albian/Santonian). The acoustic basement can be clearly seen at the base of the section.

No downhole seismic velocity logs were obtained at Site 897, so that it was not possible to correlate directly between the times of reflectors seen on the seismic reflection profiles and the various observations in cores that are referred to depth downhole. However, the results of two sonobuoy lines shot over the Iberia Abyssal Plain (Whitmarsh et al., 1990) were used to construct a two-way traveltime-to-depth conversion function that was almost identical to that derived from observations at Site 398 (Fig. 65; Sibuet, Ryan, et al., 1979). This function enabled us to estimate the depth of the reflectors seen on the Sonne seismic reflection profile across the site. An attempt was made also to calculate preliminary synthetic seismograms from the velocity and density measurements made on core samples on the ship. However, this was necessarily tentative and weakly constrained because no accurate correction was possible for the changes in velocity and density from in-situ to laboratory conditions, and the laboratory results provided a probably biased, and certainly incomplete, estimate of the in-situ values.

The part of the Sonne seismic section across Site 897 is presented in Figure 66 at an enlarged scale alongside the lithological summary and plots of bulk density, velocity, acoustic impedance, and reflection coefficient for Hole 897C. The reflectors discussed below were chosen on the basis of their continuity between Holes 897C and 897D and their amplitude. Reflector times were picked at the onset of the relevant positive pulse. One should remember that (1) the vertical resolution of the seismic profiles is approximately equal to a quarter wavelength of the predominant energy (i.e., about 15 m) and (2) the computation of reflector depth from two-way traveltime is not more accurate than 10 m.

The following correlations were done using the computed depths of the reflectors and are summarized in Table 18.

  1. Many reflectors are seen in the uppermost 300 m of the section (e.g., R1 in Fig. 66) from which cores containing sand, silty sand, and silt were obtained. These lithologies are associated with the bases of numerous turbidites, and it is likely that the contrasts in acoustic impedance at the bases of the turbidites contribute substantially to the reflection of sound. The reflectors are more likely to correspond to the net acoustic interference pattern produced by the series of turbidites than to individual turbidites that are much less than a seismic wave-length thick.
  2. Reflector R2 (associated with the acoustic formation 1A/1B unconformity computed to be at 340 mbsf), which is recognized in the vicinity of the site as a surface onto which numerous reflectors onlap, correlates with the onset at this site of a 10.5-Ma middle Miocene-late Miocene hiatus defined by the virtual absence of the nannofossil Zones NN4 to NN10. Measurements of physical properties do not suggest a physical cause for this acoustic event; it is evident principally from the angular relationship between the tilted underlying, and horizontal onlapping, reflectors.
  3. Reflector R3 has been computed to be at a depth of 420 mbsf and may correspond to the onset of claystone in cores below 440 mbsf, which is accompanied by a significant increase in density.
  4. Reflector R4 (associated with the acoustic formation 1B/2 boundary) is computed to lie at about 590 mbsf, which associates it with beds that are predominantly claystone and chalk of middle Eocene age. These beds exhibit alternating high and low velocity layers on a scale of 10 to 20 m, with the higher values exceeding velocities in the beds above 590 mbsf. This led to rapidly changing reflection coefficients, which probably explain the observed reflector.
  5. The thin unreflective zone immediately above basement (Fig. 3), which may correspond to the deposits of lithostratigraphic Unit IV, can just be discerned on the expanded seismic section between 0.675 s and basement (Fig. 66).
  6. The basement reflector was computed to lie at 670 mbsf. This places it in the lower part of lithostratigraphic Unit IV close to the depth below which no more sediment was cored.

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